Report No DWI0098

GALVANIC CORROSION OF LEAD IN COPPER PIPEWORK: PHASE II, EFFECTS OF TREATMENT ON GALVANIC CORROSION POTENTIALS

DWI0098

Dec 1985

SUMMARY

OBJECT

The work reported constituted the second phase of a DOE contract concerned with the measurement of galvanic corrosion potentials of lead solder coupled to copper. The objective of this phase, carried out during the period December 1984 to July 1985, was to test the effect of treatments to reduce the galvanic corrosion potential of four waters.

APPROACH

Among the criteria for choosing the four waters was that they should have high galvanic corrosion potentials and the water undertaking concerned would be able to carry out the necessary measurements conveniently. Treatment with silicate, sulphate, zinc and orthophosphate, and the last two in combination, were investigated on some or all of the waters; the work in the first phase had suggested that increasing their concentrations should decrease the galvanic corrosion potential. Corrosion potentials were measured using corrosion cells in a manner similar to that used in the first phase.

RESULTS

All of the treatments investigated produced a reduction in corrosion potential of about a half in the softest water investigated. In the harder waters silicate and sulphate were ineffective and the effectiveness of orthophosphate seemed to increase with time. Zinc, especially in combination with orthophosphate, was the most effective treatment for hard waters.

A commercial zinc orthophosphate corrosion inhibitor was also investigated and was found to be no more or less effective than a mixture at a similar concentration prepared from laboratory reagents. Some differences were observed in the behaviour of cells made at different times. This did not appear to be due to differences in the method of manufacture, but could have been due to differences in composition of the solder electrodes. As in the first phase, large differences were often seen in the potentials of the five cells in the same stream. A large increase in corrosion potential of one of the waters tested coincided with the replacement at the water treatment works of activated silica by a polyelectrolyte.

CONCLUSIONS

The soft water was more responsive than the hard waters to all the treatments tested. Zinc, alone or in combination with orthophosphate, was the most effective treatment overall. However, its concentration would need to be chosen carefully to produce the maximum inhibition of galvanic corrosion while avoiding deposition from solution which could lead to "dirty water problems". The optimum concentration is probably about 1 mg Zn/l, with orthophosphate also present at 0.4 mg P/l. The reduction in galvanic corrosion potential by any treatment may not be sufficiently great to eliminate the problem of high lead concentrations, especially if the corrosion potential before treatment is high or the water is relatively unresponsive.

RECOMMENDATIONS

Zinc, possibly in conjunction with orthophosphate, should be the preferred inhibitor of galvanic corrosion of lead-solder in copper pipework. Simple and cheap methods for reliable dosing of zinc need to be established.

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